It is well-known that a handover control apparatus performs handover control based on an instantaneous value of a reception level or a receive signal (strength) level of a radio wave from a communication station. An operation scheme of the conventional handover control apparatus is explained with reference to FIGS. 12 and 13.
FIG. 12 is an illustration of a situation that a vehicle C equipped with an in-vehicle terminal having a handover control apparatus moves along a route R, and FIG. 13 is a diagram showing two transition trends of reception level, a transition trend of a radio wave from a communication station B1 and a transition trend of a radio wave from a communication station B2.
In FIG. 12, the distance between the in-vehicle terminal and the communication station B1 is shorter than the distance between the in-vehicle terminal and the communication station B2. However, at some points during the travel of the vehicle C along the route R, the terminal and the station B1 are electrically blocked from each other by a building Sh interposed there between, as shown in FIG. 12. Therefore, the reception level of the radio wave received by the terminal from the station B1 changes drastically during the travel of the vehicle C along the route R. More specifically, as shown in FIG. 13, the reception level of the radio wave received by terminal from the station B1 transits at a high level between time t10 and time t11, during which no blocking building Sh exists between the terminal and the station B1, and, at time t11 the blocking of the radio wave by the building Sh starts. Then, the reception level starts to steeply decrease after time t11, to a reception level that is lower than a reception level of the radio wave from the station B2 at time t12. Further, the reception level of the radio wave from the station B1 starts to increase at time t13, to exceed the reception level of the radio wave from the station B2 at time t14, to start to transit at the high level after time t15, at which the blocking building Sh disappears from the space between the terminal and the station B1. In other words, the reception level of the radio wave from the station B1 temporarily drops due to the building Sh. On the other hand, due to no blocking by a building, a transition trend of the reception level of the radio wave from the station B2 is stable during the travel of the vehicle C along the route R, although the distance between the terminal and the station B2 is greater than the distance between the terminal and the station B1, and the reception level of the station B2 at the terminal is lower than the reception level of the station B1 at the terminal, as shown in FIGS. 12 and 13.
In the above-described situation, the conventional handover control apparatus performs handover in the following manner. After performing wireless communication with a serving station B1 between time t10 and time t12 due to the reception level of the station B1 being higher than the reception level of the station B2, the conventional handover control apparatus performs handover by switching the serving station B1 to the communication station B2 at time t12, at which time the reception level of the station B1 falls below the reception level of the station B2. Further, the conventional handover control apparatus performs handover by switching the serving station back to the communication station B1 at time t14, at which time the reception level of the station B1 exceeds the reception level of the station B2, and performs wireless communication with the station B1 that serves as the serving station thereafter.
The conventional handover control apparatus performs, as described above, handover based on an instantaneous reception level of the received radio wave from the communication station, thereby causing a frequent switching between two or more serving stations, between the station B1 and the station B2 in this case. Such phenomenon of frequent switching is called as a “roll-over.”
In addition, if we assume another station B3 having a reception level continuously higher than the reception level of both of the stations B1 and B2 after time t12, handover is performed in the following manner.
More specifically, the conventional handover control apparatus performs handover at time t12 from the station B1 to the station B2, and, immediately after the above handover, performs another handover from the station B2 to the station B3, to have wireless communication with the station B3 which serves as a serving station. The handover control scheme by the conventional handover control apparatus causes unnecessary handover due to a handover determination based on an instantaneous reception level of the received radio wave from a communication station.
Instead, one handover immediately after the other handover should rather be performed as only one handover. In the above-described case, one handover from the station B1 to the station B2 with the other handover subsequently performed from the station B2 to the station B3 should rather be one handover from the station B1 to the station B3.
When handover is performed, the wireless communication between the terminal and the communication station is temporarily interrupted, and causes an unstable wireless communication and lowers communication throughput, which is why frequent handover is not desirable.
For solving such a problem, Japanese Patent 2007-251654 (JP '654) teaches to measure the number of handovers per unit time, and changes the handover threshold that triggers handover according to the measured number of handovers. When the number of handovers performed per unit time is great, the number of handovers can be decreased by changing the handover threshold to have a lower value. As a result, unnecessary handover is prevented.
The technique in JP '654 decreases the number of handovers simply by changing the threshold, running a risk of preventing a required handover as well. For example, in an urban area where there are many blocking objects, the radio wave from the serving station to the terminal is transmitted through multiple paths (i.e., a multi-path environment). In such an environment, the instantaneous value of the reception level drastically changes when the terminal travels by only a small amount. As a result, the reception level frequently falls below the handover threshold, thereby increasing the number of handovers. Therefore, the technique in JP '654 should naturally decrease the handover threshold value in the multi-path environment, resulting to prevent a “necessary/required” handover to an adjacent station for continuation of the wireless communication with a current serving station, without regard to a decreased reception level of the current serving station. Further, an already-decreased handover threshold value described above triggers handover to the adjacent station at a timing that is too late for keeping the uninterrupted wireless communication between the terminal and the current serving station, if the decrease of the reception level at the terminal is steep.
In addition, handover may be performed based on an averaged reception level instead of the instantaneous reception level. In such modification of the handover control scheme, handover control for a period between times t12 and t14 in FIG. 13 is prevented, due to the average of the reception level of the station B1 exceeding the average of the reception level of the station B2 for the same period.
However, the transition trend of the reception level of the radio wave received from the communication station B1 may not necessarily be stable even when the averaged reception level from the station B1 for a certain reception period is high. For example, in the above modification example, handover to the communication station B2 will be performed only on condition that the averaged reception level of the station B2 for a certain period exceeds the averaged reception level of the station B1, in spite of to a drastic change of the reception level of the station B2 for the same period. As a result, the terminal cannot perform a stable wireless communication after such handover to the station B2.